The Cost
of Capital

An energy model can be physically careful and financially wrong. The load may be measured, the equipment quote current, and the operating assumptions explicit. Yet one cell can still decide the recommendation before the comparison begins: the discount rate.

Section 01The decision hidden inside the model

Every long-lived energy option trades money at different points in time. A utility extension may concentrate cost near energization. Efficiency may spend less upfront and begin reducing consumption sooner. Solar, storage, fuel cells, engines, turbines, and newer generating platforms each place construction cost, fuel, service, replacement, and residual value on a different calendar. Adding the undiscounted dollars does not make those calendars comparable.

Discounting converts future amounts to a common base date. The 2025 edition of NIST Handbook 135 describes the private-sector discount rate as the investor’s minimum acceptable rate of return for investments of equivalent risk and duration. It also notes that appropriate rates can vary significantly across investors because their alternative opportunities differ.[1] That definition matters. The right rate is not the rate that makes a favored project work. It is the owner’s documented standard for capital with comparable risk and duration.

A rate copied from a vendor model answers a different question. It may describe the vendor’s financing, a project company’s debt-and-equity structure, a regulated utility’s allowed return, or an industry benchmark. None automatically represents what this owner gives up by committing money to this decision.

Section 02WACC is useful, but it is not universal

Weighted average cost of capital combines the expected cost of debt and equity according to their shares in the financing structure, with tax effects handled consistently. The Electricity Annual Technology Baseline uses WACC as the discount-rate input to the capital recovery factor in its levelized-cost calculation. Its financial cases vary debt interest, equity return, leverage, and debt-service assumptions to reflect differences in perceived technology, construction, and operating risk.[2]

That makes WACC a disciplined framework. It does not make one published WACC transferable to every owner. A large operating company funding a project from its balance sheet, a property owner using a secured loan, and a special-purpose project financed against contracted cash flows do not have the same capital structure. They may own the same equipment and still face different financing costs.

The Department of Energy’s financing guidance states that loan term, interest rate, down payment, and other parameters depend on the customer’s credit history, the lender’s perception of default risk, and sometimes the details of the proposed project.[4] Its commercial financing primer also observes that a company may require a high minimum return because the same capital could be deployed into other high-return investments.[5] The relevant rate therefore begins with the actual decision-maker and actual source of funds.

Section 03The market provides a floor, not the answer

Market rates are observable. An owner’s complete cost of capital is not found by selecting one of them. The Federal Reserve’s August 25, 2026 H.15 release reported the following August 24 observations: a 3.63 percent effective federal funds rate, a 6.75 percent bank prime rate, a 4.70 percent nominal ten-year Treasury constant maturity, and a 2.38 percent ten-year inflation-indexed Treasury constant maturity.[3] These are sourced-and-dated market facts. They are not a recommended project discount rate.

A risk-free reference can anchor the discussion. Credit spread, project risk, construction exposure, equipment maturity, contract quality, liquidity, and the owner’s alternative uses of capital sit above or beside that anchor. Bank prime is also only a base rate. The offered loan rate, required equity, fees, collateral, covenants, and draw schedule determine the cash flows the owner will actually face.

Section 04Real and nominal must never cross

A second error can overwhelm the choice of rate: discounting one type of dollar with another type of rate. NIST presents two consistent methods. Future cash flows stated in constant dollars are discounted with a real rate that excludes general inflation. Future cash flows stated in current dollars are discounted with a nominal rate that includes inflation. With consistent assumptions, both methods produce the same present value.[1]

Mixing them biases the result. Escalating utility bills, fuel, labor, and maintenance in nominal dollars while discounting with a real rate overstates future amounts. Holding future costs in today’s dollars while applying a nominal rate understates them. A model that does not identify its dollar basis cannot be audited from the output.

The study period needs the same discipline. Competing paths should use a common base date, service date, and evaluation horizon. Shorter-lived components require replacement assumptions when the service need continues beyond their life. Longer-lived assets may retain residual value at the end of the study. Truncating the model just before a replacement or extending it only far enough to capture a favored asset’s later benefits is not neutral analysis.

Section 05One project, three financial answers

Consider an explicitly illustrative project. It requires an $8 million payment at the base date and produces $1 million of net owner cash benefit at the end of each year for 20 years. This is arithmetic only. It is not a Bcal cost estimate, savings forecast, equipment case, or offer.

The physical project did not change. The timing did not change. The rate changed the present value of later cash flows enough to reverse the sign of the result. That is why a model should never present one rate without a reason and a sensitivity range.

The illustration also shows why long-duration benefits are especially sensitive to capital assumptions. A project whose value arrives late loses more present value as the rate rises than a project whose value arrives early. This does not make short-payback projects inherently better. It means timing is part of the product being purchased.

Section 06Do not hide every risk inside the rate

Risk can be reflected in the discount rate, in the cash flows, or in explicit scenarios. Using all three without discipline double-counts it. If expected output has already been reduced for degradation and availability, then adding a large unexplained risk premium for the same uncertainty penalizes the project twice. If construction delay is modeled as a later service date and higher carrying cost, another generic delay premium may repeat the same adjustment.

The opposite failure is just as common. A base-case cash flow may assume timely construction, full performance, stable fuel, predictable maintenance, and complete residual value, while the rate remains close to a low-risk market benchmark. The spreadsheet then treats uncertain cash flows as if they were contracted.

A better structure separates the questions:

• Rate: What return does the owner require for capital of comparable risk and duration?
• Cash flow: What amount is expected, when does it occur, and which party is obligated to pay or perform?
• Scenario: What happens if energization, production, fuel, service cost, replacement, or residual value differs from the base case?
• Break-even: At what value does the recommendation change?

NIST treats sensitivity and break-even analysis as accessible ways to expose critical inputs, while warning that sensitivity results do not state how likely each outcome is.[1] That warning belongs in the board memo. A range is not a probability distribution.

Section 07The technology-neutral capital test

Cost of capital does not select a technology by itself. It changes the weight placed on timing, durability, replacement, fuel, and contractual evidence. Every path deserves the same base date, rate convention, study horizon, and disclosure standard.

A lower discount rate tends to give more weight to benefits that arrive farther in the future. A higher rate gives greater relative weight to early cash flows and near-term optionality. Neither rate is automatically conservative. A low rate can overvalue uncertain long-dated benefits. A high rate can understate durable infrastructure value and favor temporary measures that require repeated replacement. The defensible approach is to disclose the owner’s rate, then show whether the ranking survives reasonable alternatives.

Section 08Set the rate from evidence

The rate-setting conversation should happen before vendor economics are compared. Finance, operations, facilities, and the economic buyer may each hold part of the answer. The owner should identify whether the project competes with ordinary maintenance capital, growth capital, a real-estate investment, debt reduction, or another operating project. Comparable risk and duration matter more than a generic corporate label.

Where a specific loan is contemplated, use the actual indicative terms: rate basis, spread, fees, tenor, amortization, down payment, collateral, covenants, and draw timing. Where balance-sheet capital is used, document the approved hurdle rate or capital-allocation rule. Where a project structure combines debt and equity, show the weights and required returns rather than presenting WACC as an unexplained output.

If the rate is not yet known, the study should not invent precision. It can present a sourced market reference, an explicitly illustrative range, and the break-even rate at which the preferred option changes. The missing owner decision remains labeled as missing. That is more useful than a single decimal carried through twenty years of cash flow.

Section 09The disclosure block every study needs

1. Name the decision-maker.Identify whose capital standard governs and who can approve the investment.
2. State the rate and basis.Show the discount rate, nominal or real treatment, base date, and inflation convention.
3. Show the capital structure.Separate debt, equity, fees, tax effects, and construction-period financing where they apply.
4. Use one comparison horizon.Align service dates and study periods; include replacements and residual values consistently.
5. Keep risk visible.Identify which risks are in expected cash flows, which are scenarios, and which affect the required return.
6. Publish sensitivity and break-even results.Show the rates that change the ranking and the assumption that becomes decisive.
7. Tag every figure.Distinguish sourced-and-dated facts, current quotes, and clearly labeled illustrative estimates.

This block lets a reviewer reproduce the financial logic without accepting the recommendation on faith. It also prevents a late change in the hurdle rate from appearing as a technical failure. If the owner changes its capital standard, the model can change with it and preserve the audit trail.

Section 10The rate is a governance decision

The discount rate does not belong to the technology vendor. It belongs to the owner’s capital process. The analyst’s job is to apply it consistently, expose its effect, and separate it from the physical and contractual risks already modeled elsewhere.

Sometimes the ranking will be stable across every reasonable rate. That is strong evidence. Sometimes two options will cross within the owner’s plausible range. That is not a defect; it identifies the exact capital judgment leadership must make. Sometimes the no-project case will win because waiting preserves more value. That outcome belongs in a technology-neutral study too.

The honest recommendation is therefore conditional and written: this path leads under this documented capital standard, with these cash flows, over this period, and it changes when this assumption crosses this threshold. That is what makes the financial answer usable after the meeting where the spreadsheet was first shown.